This disclosure relates generally to the fuel cell field, and more particularly to a fuel cell system with anode protection and a shutdown method for the fuel cell system.
Fuel cells are electro-chemical devices which can convert chemical energy from a fuel into electrical energy through an electro-chemical reaction of the fuel, such as hydrogen, with an oxidizer, such as oxygen contained in the atmospheric air. Fuel cell systems are being widely developed as an energy supply system because fuel cells are environmentally superior and highly efficient. To improve system efficiency and fuel utilization and reduce external water usage, the fuel cell system usually includes an anode recirculation loop. As single fuel cell can only generate 1V voltage, therefore, a plurality of fuel cells are usually stacked together (usually referred to as a fuel cell stack) to get desired voltage.
An anode of a typical solid oxide fuel cell (SOFC) is commonly made of a nickel/yttria-stabilized zirconia (Ni/YSZ) cermet. Nickel in the anode serves as a catalyst for fuel oxidation and current conductor. During normal operation of the fuel cell system, SOFC stacks are typically operated at above 700° C., and the nickel (Ni) in the anode remains in its reduced form due to the continuous supply of primarily hydrogen fuel gas.
However, for example, when crossover or overboard leakage occurs in the fuel cell stack, if the reducing gas is not adequate in the anode, the Ni in the anode may undergo a re-oxidation, where the Ni may react with the oxygen in the air diffused from the cathode layer or introduced into the anode chamber to form nickel oxide (NiO) at temperatures above approximately 350° C. The formation of NiO in the microstructure of the anode may result in volumetric expansion of the anode layer, which exerts stress on the overall SOFC structure. During rapid oxidation, the electrolyte is unable to expand as fast as the forming nickel oxide, resulting in the potential to crack the electrolyte. This will allow the fuel and oxidant gases to mix directly, which may lead to catastrophic results if the fuel cell temperature is above the auto-ignition temperature of the fuel.
In a laboratory setting, the SOFC stack may be protected from re-oxidation using a supply of reducing gas, which is typically a dilute mixture of hydrogen in nitrogen gas. This can be used to purge the anode chamber during SOFC shutdown or standby conditions to prevent re-oxidation of the anode. A typical SOFC stack requires usually between four to twelve hours cooling from its operating temperature to a temperature below which there is no significant damage to the anode material can occur. During this time, it will require a large amount of reducing gas and frequent bottle changes to meet the reducing gas consumption demand.
Furthermore, during shutdown process with fuel cell stack leakage, there is a tendency for carbon formation and deposition on anode electrodes. The carbon could be formed by hydrocarbon cracking, Boudouard reaction or carbon monoxide (CO) reduction. The carbon could be removed by introducing steam to the fuel cell system which leads to CO and Hydrogen at certain conditions. The carbon will be deposited if the carbon formation rate is faster than the carbon removal rate. Internal carbon formation and deposition results in decreased efficiency of the anode, reduced useful device lifetime and forced shutdown of the system.
Therefore, there is a need for an improved fuel cell system to prevent the oxidation of nickel and carbon deposition in the anode of the fuel cell stack during shutdown operation. There is a further need for this system to be economical to install and operate.